In a plasma processing apparatus which performs a processing such as a plasma processing on a semiconductor wafer, a substrate accommodated in a container is transported by a transport robot to a plasma processing chamber maintained in a vacuum atmosphere. Generally, the plasma processing apparatus includes a plurality of plasma chambers and one transport robot is provided to access each of the plasma chambers.
Here, in order to save the space in arranging a plurality of plasma chambers, enable the access to each of the plasma chambers by one transport robot, and shorten the transportation distance or transportation time of the semiconductor wafer for the purpose of enhancing the throughput, it is required to change the travel direction during the transportation of the semiconductor wafer. At that time, it is required to control the operation of the transport robot such that the semiconductor wafer moves along a smoothly curved trajectory.
As a method for calculating the transportation path of a substrate such as a semiconductor wafer, a so-called circular interpolation method is utilized. For example, as a method of performing a circular interpolation between a start point and an end point with setting a distance from the start point to the center point as a radius, in a case of calculating a difference between a distance from the end point to the center and the radius, and performing a circular interpolation from the start point to the end point to obtain points on a circular arc, there is suggested a method of interpolating the calculated differences to correct the radii at each interpolation point, and obtaining coordinates of each interpolation points based on the corrected radii. See, e.g., Japanese Patent Laid-Open No. H4-123204.
In addition, there has been known a circular interpolation method of calculating a circular arc which contacts at a certain distance (pass strength) from an intersection point of two straight lines indicating a travel direction. By this circular interpolation method, a method for determining a transportation path transporting a substrate is schematically illustrated in FIGS. 3A to 3D.
FIG. 3A is a schematic view illustrating a substrate transportation path determining method in the related art. A transportation initiating point of a substrate S is set as a start point O, a transportation terminating point of the substrate S is set as an end point P2, an intersection point of straight lines “a” and “b” indicating two travel directions is set as a virtual via-point P1, and these points O, P2, P1 are predetermined. Further, the expression “virtual via-point” is used herein because the substrate S does not pass along the virtual via-point P1, but actually passes along a calculated circular arc “a”, as described below.
By calculating the circular arc “a” in contact with the straight line “a” at a distance C from the virtual via-point P1 and in contact with the straight line “b”, a transportation path is determined, which travels from the start point O, passes through a position P3 (a contact point of the straight line a and the circular arc a), the circular arc “a” and a position P4 (a contact point of the straight line “b” and the circular arc “a”), and arrives at a position P2.
FIG. 3B illustrates an example in which the substrate transportation path determining method of FIG. 3A is applied to a substrate transportation path determining method in a case where a transport robot carries a substrate into a disposing chamber. A fork 93 installed at the tip of an arm (not illustrated) of the transport robot holds the substrate S, and carries the held substrate S into a disposing chamber 92. Here, it is assumed that the center of the substrate S is consistent with the center of the fork 93, and a gate valve 91 is installed at a substrate carrying-in/out port to open/close the substrate carrying-in/out port.
The straight line “a”, the straight line “b”, the circular arc “a”, the virtual via-point P1 and the position P2 as illustrated in FIG. 3B correspond to those of FIG. 3A, respectively. The center of the gate valve 91 is set as the virtual via-point P1, and the center of the disposing chamber 92 is set as the position P2. The straight line “b” passing through the virtual via-point P1 and the position P2 represents a transportation direction carrying the substrate S straightly with respect to the disposing chamber 92, and the straight line a represents a transportation direction of the substrate S determined depending on the disposal position of the transport robot and the disposing chamber 92.
By controlling the operation of the transport robot such that the center of the fork 93 passes through the straight line “a”, the circular arc “a” and the straight line “b”, the substrate S may be transported smoothly in a short path. Further, in a case where the substrate S is transported by passing through the virtual via-point P1 along the straight line “a” and the straight line b, there is a concern that the transportation direction may be suddenly changed at the virtual via-point P1, and thus, a positional deviation may be caused by an inertial force on the substrate S held in the fork 93. However, since the substrate S is transported so as to pass through the circular arc “a” without passing through the virtual via-point P1, the positional deviation of the substrate S with respect to the fork 93 may be suppressed.